LATE QUATERNARY VEGETATION HISTORY OF THE KEEP RIVER NATIONAL PARK, NORTHERN TERRITORY, AUSTRALIA

A 30,000 year record of paleoenvironmental change has been derived from the stable isotope composition of the bulk organic carbon fraction and lipid biomarkers extracted from a core taken from a mound spring at the Keep River National Park, Northern Territory, Australia. The bulk sediment carbon isotope data represent a mix of C₃ and C₄ plants between 30,000 and 12,000 ¹⁴C yr BP, (d¹³C values range between -28 and -14‰) and nearly 100% C₃ plants between 9,000 14C yr BP and present (d¹³C values approximate -28‰ for the time period).

The carbon isotope composition of the long chain fatty acids (d¹³C_LCFA) diagnostic of higher plants (C24, C26, C28) range between -25 and -27 ‰ from 30,000 to 18,000 ¹⁴C yr BP, indicating that the region was dominated by C₄ plants (grasses, and possibly sedges). After 18,000 ¹⁴C yr BP, the d¹³C_LCFA decrease to approximately -35 ‰ by 9,000 ¹⁴C yr BP and remain constant to the present day, indicating that the region was composed of a 50-50 mix of C₃ and C₄ plants.

There is good agreement between the bulk sediment and compound specific isotope data between 30,000 and 9,000 ¹⁴C yr BP; C₄ plants dominate the organic input to the site. During the Holocene, however, there is an apparent discrepancy between the bulk and compound specific isotope data that can be explained by a shift in depositional setting at the site. As the mound spring becomes established and elevated relative to the surrounding plains (beginning at 9,000 ¹⁴C yrs BP), fewer materials can be washed in from the surrounding environment. The bulk sediment isotope data represent organic deposition at the mound-spring proper (i.e., 100% C₃). The n-fatty acids can be transported via wind and/or water, and therefore represents a mix of local (i.e. C₃) as well as distal (i.e., C₄) plant sources.

The fatty acid distributions and the d¹³C_LCFA values indicate the establishment of a reliable source of water (i.e., the mound spring) at the site by the early Holocene. These fatty acid and isotopic data provide evidence for a shift from arid and/or cooler conditions during the Last Glacial Maximum (21,000 to 18,000 ¹⁴C yr BP) (i.e., C₄ grass dominance) to consistently wetter conditions by the early Holocene (10,000 ¹⁴C yr BP) (i.e., C₃ vegetation at the site surrounded by C₄ grass plains) in response to enhancement of the Australian summer monsoon.